A. Field of the Invention
The invention relates to glassy metal alloy compositions, and, in particular, to compositions including substantial amounts of one or more of the refractory metals of molybdenum, tungsten, tantalum and niobium. The glassy compositions of the invention evidence a combination of high crystallization temperatures and high hardness values.
B. Description of the Prior Art
Investigations have demonstrated that it is possible to obtain solid glassy metals for certain alloy compositions. A glassy substance generally characterizes a noncrystalline substance; that is, a substance substantially lacking any long range order. In distinguishing a glassy substance from a crystalline substance, X-ray diffraction measurements are generally suitably employed. Additionally, transmission electron micrography and electron diffraction can be used to distinguish between the glassy and the crystalline state.
A glassy metal produces an X-ray diffraction profile in which intensity varies slowly with diffraction angle. Such a profile is qualitatively similar to the diffraction profile of a liquid or ordinary window glass. On the other hand, a crystalline metal produces a diffraction profile in which intensity varies rapidly with diffraction angle.
These glassy metals exist in a metastable state. Upon heating to a sufficiently high temperature, they crystallize with evolution of a heat of crystallization, and the diffraction profile changes from one having glassy characteristics to one having crystalline characteristics.
It is possible to produce a metal which is a two-phase mixture of the glassy and the crystalline state; the relative proportions can vary from totally crystalline to totally glassy. A glassy metal, as employed herein, refers to a metal which is primarily glassy, but which may have a small fraction of the material present as included crystallites. Substantially glassy metals are preferred, due to an increase in ductility with an increase in glassiness.
For a suitable composition, proper processing will produce a metal in the glassy state. One typical procedure is to cause the molten alloy to be spread thinly in contact with a solid metal substrate, such as copper or aluminum, so that the molten metal rapidly loses its heat to the substrate.
When the alloy is spread to a thickness of about 0.002 inch, cooling rates of the order of 10.sup.6 .degree. C./sec may be achieved. See, for example, R. C. Ruhl, Vol. 1, Materials Science & Engineering, pp. 313-319 (1967), which discusses the dependence of cooling rates upon the conditions of processing the molten metal. For an alloy of proper composition and for a sufficiently high cooling rate, such a process produces a glassy metal. Any process which provides a suitably high cooling rate can be used. Illustrative examples of procedures which can be used to make the glassy metals include rotating double rolls, as described by H. S. Chen and C. E. Miller, Vol. 41, Reviews of Scientific Instruments, pp. 1237-1238 (1970), and rotating cylinder techniques, as described by R. Pond, Jr. and R. Maddin, Vol. 245, Transactions of Metallurgical Society, AIME, pp. 2475-2476 (1969).
Glassy alloys containing substantial amounts of one or more of the transition metals of iron, nickel, cobalt, vanadium and chromium have been disclosed by H. S. Chen and D. E. Polk in U.S. Pat. No. 3,856,513, issued Dec. 24, 1974. Such alloys are quite useful for a variety of applications. Such alloys, however, are characterized by a crystallization temperature of about 425.degree. C. to 550.degree. C. and a hardness of about 600 to 830 kg/mm.sup.2.